1. Field of the Invention
The present invention generally relates to overcurrent protection for circuit interrupting devices.
2. Description of the Related Art
Circuit interrupting devices are typically designed to safely provide electrical power to an electrical circuit or system to which such devices are electrically coupled. A circuit interrupting device has line terminals that are connected to a power source and load terminals that are connected to one or more electrical devices or systems. The electrical devices and systems are commonly referred to as loads. The line terminals usually comprise a line phase terminal (Lø) and a line neutral terminal (LN). Similarly, the load terminals usually comprise a load phase terminal (LDø) and a load neutral terminal (LDN).
During operation, a circuit interrupting device provides a conductive path through which electrical current is routed from the line phase terminal to the load phase terminal and to the load. The circuit interrupting device provides a return path for the current whereby the current is routed from the load to the load neutral terminal and back to the power source via the line neutral terminal. The line neutral terminal is usually connected to a reference point commonly referred to as system ground. The circuit interrupting device is triggered when a current imbalance exists; that is, when the current into the circuit interrupting device (i.e., current through the line phase terminal) is different in magnitude than the current exiting the device (i.e., current through the line neutral terminal). The cause of such a current imbalance is typically due to leakage current at the load or leakage current somewhere along the return path of the current from the load. In other words, some of the current flowing into the load is routed to some path (i.e., leakage path) other than the return path. The leakage path can be located at the load or somewhere along the return path.
Referring to FIG. 1, there is shown a circuit interrupting device 100 having a current sensing device 102 and a switch SW1 comprising switches SW1A and SW1B. When IIN (input current) is substantially equal to IOUT (output current), the switches are closed by current sensing device 102 allowing current to flow from a power source (not shown) to circuit interrupting device 100 and onto a load 104. There are two examples of current leakages shown. As stated above, current leakage at load 104 occurs when some of the current flowing into the load returns to earth ground through an alternate path. In actual cases, the alternate path may be thru a person in which case the flow of current must be stopped as quickly as possible in order to avoid serious injury or even death to such a person. The current leakage at the load is shown as ILEAK1. Current leakage along the return path is shown as ILEAK2. Although not shown, there can be leakage current before the load.
When current sensing device 102 detects a current imbalance whereby IIN≠IOUT, it causes switches SW1A and SW1B to open thereby terminating the flow of current at the load phase and load neutral terminal. Current sensing device 102 can be designed to detect relatively small differences between the input and output currents. Current interrupting device 100, once triggered, can be reset and once again be in a state to detect current imbalances. The resetting of circuit interrupting device 100 is usually with a pushbutton switch readily accessible to a user of such a device.
Although current interrupting devices are designed to detect current imbalances, they are not designed to detect overcurrent situations. Overcurrent situations are scenarios where a device is drawing a relatively large amount of current that can cause permanent or serious damage to such device. For example, an electrical short within a load connected to the circuit interrupting device can cause a relatively large amount of current to flow through the load and also through the circuit interrupting device to which the load is connected. The large current surge not only can damage the load, but can also damage the circuit interrupting device. Typically, the load is provided with a fuse rated at a certain current value such that when the current through the load attempts to surpass the rated current value, the fuse is permanently disabled (e.g., fuse filament is burnt) preventing any further current flow from the source to the load.
The current interrupting device may or may not have a fuse. If the circuit interrupting device is designed with a fuse, the current rating of such a fuse may not match the current rating of the load. For example, if the current rating of the load is more than the current rating of the current interrupting device, the fuse of the current interrupting device may be blown for current which are acceptable to the load; this situation will cause an inefficient operation of the device. Notwithstanding the current rating mismatch between the load and the current interrupting device, whenever a fuse is permanently damaged, there may be required some physical disassembly of the load device and/or the circuit interrupting device.
Currently, the approaches to resolving overcurrent situations are inadequate and burdensome. A circuit interrupting device which can protect itself and the load from overcurrent situations is desirable. Furthermore, standards bodies and organizations such as Underwriters Laboratories (UL) are leaning towards requiring that circuit interrupting devices be designed with overcurrent protection.